Dichromated gelatine (DCG) is a well known material in the holographic field. It is customarily used in the manufacture of head-up displays for military aircraft. It is a gelatinous material with usually dissolved dichromate (sodium, potassium or ammonium for example). Typically a layer of dichromated gelatine is provided on a transparent substrate such as glass or transparent plastics, and is exposed and processed on that substrate. The material works on the basis that where light falls a hardening reaction takes place in the gelatine involving cross-linking of the gelatine's protein structure. This cross-linking is, of course, spatially variable and leads to highly localised stresses in the gelatine layer. With appropriate processing the result is a material composed of gelatine with air-voids within its structure. The refractive index differential (An) between different parts of the layer (which have been exposed to different amounts of light), can be very high (up to 0. 3).

Advantages of DCG are its low cost and the ability to record on a very short time scale. Although materials with greater sensitivity to light exist, the reaction occurring on exposure to appropriate selected laser light is very rapid.

In some embodiments of the present invention, DCG is advantageously exposed by laser scanning, i. e. by scanning a DCG layer by means of a laser beam caused to scan the material in a raster. This technique may be used to record in the DCG various desired graded refractive index structures, such as arrays of microlenses or analogous structures. Existing mask making technology, such as disclosed, for example, in EP-A-0294122 or US-A-5695895 may be used to produce optical masks through which a DCG layer may be exposed to laser light from a laser beam caused to scan the DCG layer, through the superimposed mask, in a scanning raster, in a moderately fast, but direct, recording process. Typically the DCG would be sensitive to about 300 mJ cm-) of 488 nm argon laser light. Alternatively, the dichromated gelatine. with the superimposed mask, could be subject to a blanket exposure, through such mask, to light (not necessarily laser light), of an appropriate wavelength.

One possible application of DCG material in accordance with the invention is in the production of diffusers, for example diffusers with unusual angular properties. Such diffusers may be produced by exposure to light of a coating or layer of DCG on a transparent support, e. g. a glass plate, through an optical mask in the form of an opaque mask with an array of apertures therethrough for the passage of light.

The scattering of light by such a diffuser is an achromatic process and as the diffusion effects are kept within the volume of the material, then extremely unusual diffusers can be produced in accordance with the invention. For example, a deviating diffuser can be recorded by the use of, for example, intelligent angling of a recording laser beam, and which will act as a combined thin prism and diffuser, i. e. which will scatter light in such a way that the mean direction of the light scattered by the diffuser is inclined somewhat with respect to the direction of the incident light. The applicants have found that such a deviating diffuser can be produced by exposing DCG, with a superimposed aperture mask, to a beam of light directed onto the DCG layer/mask combination, along an axis which is parallel to a predetermined axis inclined with respect to the normal to the plane of the DCG layer and mask, i. e. so that the light passing through each mask aperture into the DCG material passes parallel with said predetermined axis. Naturally, slight deviation from strict parallelism of different parts of said beam, (or of said beam at different points in its scan, where a scanning beam is employed), with said predetermined axis will not destroy the mean deviating effect. As an example of the attractive possibilities in accordance with the invention. large-ara micro-diffusers with such mean deviation property may be used in the windows of buildings where incoming sunlight would be best diverted to light the walls of the room instead of less usefully lighting the floor. As a second example, a single DCG diffuser panel might be produced which might offer different pictorial images to the left and right eyes of an observer, thus leading to a very pragmatic version of a stereoscopic television system, in a manner similar to the stereoscopic television system, utilising photopolymer micro-lens arrays, disclosed in EP-A- 0294122.

As noted above, dichromated gelatine (DCG) is a gelatine based material with a photosensitive salt in it. In some embodiments of the invention, the gelatine used may be mixed with a polymer. Whilst, in most embodiments of the invention, the salt added to the gelatine or gelatine/polymer mix would be a dichromate, so that the material would be dichromated gelatine, other salts could conceivably be used and the term"dichromated gelatine"is to be construed correspondingly broadly herein. In carrying out the invention, the dichromate is added in an amount depending upon the thickness of the gelatine film (10p to 100) used and the wavelength of light used for exposure. Typically the range would be 1% to 10% by wt. dichromate but preferred examples have used 6% for both 3 and xi film thicknesses.

The applicants have noted that the DCG material may produce reflection of light using total internal reflection at refractive index discontinuations in the material itself caused by cross-linking of the gelatine where the light falls on it.

Preferably, in carrying out the present invention, a DCG layer or film is subjected to a pre-exposure to light, which may be a blanket exposure, before the selective main exposure, e. g. through an optical mask, to produce the desired refractive index variations in the material. Exposure of the film to incoherent light of the chosen frequency prior to the main exposure through a mask or by selective scanning helps to free ions (usually of CR3 +) and this enables shorter main exposure timing which can help processing economics.

Typically such pre-exposure may be effected by passing the DCG film over an UV tube before the optical mask is applied to the DCG film and the mask/DCG combination scanned with an UV laser.

The selective, main exposure of the DCG is typically to light of an intensity in the range of 300 to 500 mJ/cm2 depending upon the strength of volumetric diffusion of light required. The tendency is that very high diffusion reduces the off-axis effect. The relative humidity at which exposure takes place is more important than temperature. Low humidity is preferred for processing. The "bluer"the light source the finer the feature details which can be copied from the mask so that UV is preferred. However UV light is not essential and the light from an argon laser can be usefully employed as it contains two wavelengths and so has a stronger effect on exposure. Whilst direct laser scanning is possible with the location of graded refractive index features being determined for example with selective energisation of the laser during the scan or selective gating of the laser beam onto the target during the scan, for maximum control it is better to use a pre-prepared chrome mask with randomised"holes"or"spots". One viable scheme is to use a mask having features (i. e. "holes"or"spots") of three (standard) sizes and shapes, with randomisation being achieved by randomising the features both by mix and position on the mask. Such randomising will eliminate any diffraction effects as per Young's Eviometer. The mean feature size should preferably by around 1/lOth of the film thickness (i. e. 3 for a 30 thickness of film). The density of the mask can be as low as 2 (equivalent to 1% transmittance) but is preferably around 0.7 (equivalent to over 50% transmittance). The mask is preferably truly binary (i. e. consisting of totally clear regions and totally opaque (black) regions) and feature size may be limited by emulsion thickness.

It is envisaged that in commercial production, the unexposed material, comprising a dry DCG layer on a plastics film (which may be rolled up) or on a glass plate or the equivalent will have the mask superimposed thereon, ideally using a vacuum chuck. Thus the material in flexible film form may be applied by vacuum onto a roller and fed continuously longitudinally over the roller during continuous production. Alternatively film or plate material may be processed by a successive"step and expose"procedure. The exposure is preferably undertaken using a suitable laser or other sensitiser source of visible or UV light. In preferred embodiments of the invention, the DCG material after exposure is transferred to a bath containing 50g of sodium sulphite (anhydrous) and lg of potassium hydroxide in 1 litre of water (distilled by preference). The immersion time is about 2 minutes at ambient (20°C) temperature. This is a softening step which is the reverse of normal practice. Where a product is destined to form a rear projection screen or a depixelating screen, for example, it is also possible to include in the bath a cold water dye to endow the film layer with a neutral density that will increase the visual contrast of a displayed image under high ambient lighting. If possible a toner could be used instead of dye to give a more durable colour under high light conditions. Preferably the colour chosen will be grey. After immersion in the above-noted bath, the material is removed from the bath and is washed thoroughly in running water for a period of between 5 and 10 minutes depending upon the thickness of the DCG layer. The material is then placed in a bath of 50/50 propanol and water for around 10 minutes (the time again depends upon the thickness of the DCG layer).

The material is then transferred from the 50/50 propanol/water bath to a bath of 100% propanol for around 10 minutes. The material is then removed from the 100% propanol bath and is placed in a bath of pure methyl ethyl ketone (MEK) at up to 40°C for 3 minutes to give the material a chance to attain the same temperature as the bath so that self-cooling does not cause optical degradation at the surface. This step removes the propanol and any residual water and highly strengthens the dispersion effect. It is possible to add a silane to this final drying bath in order to endow the final product with low sensitivity to water ingress.

The material is finally air-dried in an oven to make it impervious to moisture, for example, at 140°C for several hours if on glass or at 70°C for an hour or so, if on a plastic substrate.

In the techniques disclosed herein, direct exposure of the DCG to imaging light may be replaced by or supplemented by a silver halide sensitised gelatine (SHSG) technique carried out as follows:- A conventional photographic plate or film comprising a transparent substrate having a coating of conventional silver halide/gelatine emulsion thereon is exposed photographically by normal imaging or contact printing techniques to the desired image and is developed in the normal way. The developed material, in which the recorded image is defined by a corresponding distribution of silver grains, is then bleached, using a dichromate bleach known per se. The bleaching process not only removes the silver grains, but results in a higher concentration of dichromate ions in the regions of the gelatine layer previously densely populated by silver grains than in the regions of the gelatine layer previously less densely populated with silver grains or in which silver grains were absent. As a result, the material thus processed, when dried, exhibits optical properties very similar to those produced by direct exposure of uniformly dichromated gelatine to an optical image in light of the appropriate wavelength. Thus, the end product is again, a substantially transparent layer on the transparent substrate, which layer, nevertheless, exhibits variations in refractive index, in accordance with the optical image concerned, (for example an array of apertures in an otherwise opaque contact printing mask). The refractive index variations, in the material produced by the alternative technique just described, appear to result from the production of micro-voids selectively in the exposed regions, just as in the material produced by direct exposure of DCG.

The alternative technique described, producing SHSG, has the advantage of producing much enhanced sensitivity during the original imaging exposure.

Thus, for example, whilst direct exposure of DCG may require an exposure intensity of 300 millijoules per square centimetre, in the exposed regions, for adequate imaging, by using the SHSG technique, a mere 300 microjoules per square centimetre may be required. The alternative SHSG technique is more appropriate where relatively thin gelatine layers are involved, as is normally the case with photographic emulsions, whereas the direct exposure of dichromated gelatine to imaging light is the preferred technique where relatively thick layers (50 microns or greater) of DCG are to be utilise.

It is envisaged that continuous production of, for example, optically diffusing screen material utilising an SHSG technique, would involve the initial production of a silver halide/gelatine emulsion layer on a transparent plastics film, exposure of that emulsion to the desired optical image, (for example, through an optical aperture mask using laser scanning), followed by normal photographic development, followed by a bleaching step in which the material is bleached by a dichromate bleach, after which the material, possibly after further processing, may be dried and cut to size, etc. A manufacturing plant utilising this process may ideally utilise full silver recovery, (from the used bleaching solution), allowing economic utilisation of the SHSG technique.

Whilst the gelatine may be"dichromated"by adding the dichromate salt to a gelatine solution before coating a substrate with the solution and allowing the coating to dry, the gelatine coating may be formed first and the coated substrate subsequently soaked in a dichromate solution, in manner known per se. Thus it is known to provide a recording medium for phase holograms, for example, by applying a gelatine solution to a glass substrate to form a film on the substrate, after which the film is allowed to dry and is subsequently soaked in a solution of ammonium dichromate, (sodium dichromate and/or potassium dichromate may alternatively be used), and the plate thereafter dried to form the desired light-sensitive recording medium. Typically, the glass plate, with its dried gelatine coating, may be hardened by soaking in a photographic hardening solution, known per se, before washing and soaking in the dichromate solution.

The DCG coated plate is then, in the same way as described previously, exposed to a light image or pattern and is subsequently"developed"by various chemical treatments to provide a gelatine layer which is characterised by marked local variations in refractive index corresponding substantially to the image to which the plate was originally exposed.

The actual mechanism involved in the production of the observed refractive index variations in exposed and appropriately processed DCG does not appear to be fully understood, but it is known that such an exposed and developed DCG layer is characterised by minute voids or pores, which are more numerous and/or are larger in the areas not exposed, or less exposed, to light, and which are less numerous or non-existent in the areas which have been exposed to light. In some of its aspects, the present invention utilises these minute voids or pores directly. For the purposes of these aspects of the present invention, it is relevant only that this known technique makes it possible to produce, in a defined layer, relatively porous regions and relatively non-porous regions in accordance with a predetermined pattern, (predetermined by the image to which the dichromated gelatine is exposed). As already noted, in some embodiments of the invention, this image may conveniently be provided, for example, by a contact printing mask comprising, for example, an array of opaque spots in a light-transmitting or transparent field, although it will be understood that the configuration of the array and the form and configuration of said spots may be devised as desired according to the intended application.

Thus, in one embodiment of the invention these pores or voids are filled with a liquid crystal medium, the DCG layer incorporating such liquid crystal medium being, for example, sandwiched between glass plates having, in conventional manner, transparent electrodes applied thereto on their surfaces facing the DCG layer and the liquid crystal medium. In this embodiment, of course, some transparent porous medium other than DCG may be used. In another embodiment of the invention such pores or voids in DCG or some other transparent non-magnetic porous medium, for example in a thin layer on a supporting substrate, may be filled with a ferro-magnetic medium, to afford an improved magnetic record carrier such as a computer hard disk platter.

Embodiments of the invention, and methods utilising the invention, are described below by way of example with reference to the accompanying drawings, wherein:- FIGURES 1 to 4 are schematic views, in section perpendicular to the major plane of the product, illustrating successive stages in the manufacture of a light- modifying structure in accordance with the invention, FIGURES 5,6 and 7 are schematic sectional views, similar to Figures 1 to 4 but on a larger scale, illustrating a hypothetised mechanism involved in the production of a light-modifying structure in accordance with the invention, FIGURES 8 to 11 are schematic views, in section perpendicular to the major plane of the product, illustrating successive stages in the manufacture of a liquid crystal display in accordance with the invention, FIGURE 12 is a schematic fragmentary view, in section perpendicular to the plane of the platter, of a magnetic disk platter in accordance with the invention, FIGURE 13 is a schematic diagram illustrating a technique for the production of a diffusive lens in accordance with the invention, and FIGURE 14 illustrates, to an enlarged scale, a portion of a diffusing or depixelating screen in accordance with the invention.

Although it is not intended to thereby limit the scope of the invention in any way, a hypothesis as to the mechanism involved in the production of voids or pores in DCG is set out below in conjunction with disclosure of techniques which may be used in accordance with the invention, by reference to the accompanying schematic diagrams referenced Figures I to 7, which are schematic views in section perpendicular to a dichromated gelatine plate at various stages in processing.

The following description relates to the manufacture of a light-diffusing or depixelating screen made by exposure of a dichromated gelatine-coated plate, in the manner described broadly above, to an image, typically in ultraviolet light, defined by an optical contact mask placed over the dichromated gelatine layer before exposure of the layer, through the mask, to ultraviolet light. For the manufacture of the light-diffusing or depixelating screen proposed, the mask has the form of a transparent, e. g. glass, plate, carrying an array of opaque spots. Such a mask may be made, in manner known per se, by etching a glass plate provided with a thin chrome coating on one surface thereof.

Thus, Figure I illustrates the transparent glass or plastics substrate 10 after coating with a gelatine layer 12 and treating the gelatine layer with dichromate.

The layer 12 of dichromated gelatine (herein referred to as DCG) on the substrate is typically prepared by coating the substrate, in manner known per se, with a warm, fluid, gelatine solution, allowing the coating to solidify and dry, subsequently soaking the coated substrate in a solution of a dichromate salt, such as ammonium dichromate, and allowing the substrate and coating to dry again. In the preferred embodiments of the present invention, as noted above, the dichromate coating is then given a blanket exposure to light, and subsequently is exposed to light in a predetermined pattern, for example by contact printing through an optical mask 14, as illustrated in Figure 2. The mask may, as illustrated, comprise a glass plate having opaque regions 15 on its underside provided by the remnants, (after etching) of a chromium coating.

The exposed plate, after removal of the mask, is then subjected to a "development"process in which it is treated to a succession of chemicals, such as hydrogen peroxide, alkalis etc., which causes the structure of the DCG to be modified in such a way that it develops a plurality of minute pores or cracks extending from the free surface of the DCG layer in localised regions indicated at 17 by denser shading in Figure 3. The uppermost surface of the DCG layer may be covered by an imperforate transparent layer 16, as illustrated in Figure 3, for example by"flowing"the gelatine surface as described below, or by a separately formed foil or by a coating of a material which dries or cures to form an impermeable coating.

Considering in more detail the effect of the exposure, and the subsequent processing, as a result of the selective exposure to light of the dichromated gelatine coating, e. g. exposure through an optical contact mask 14,15, to ultraviolet light, in the stage illustrated in Figure 2, the regions of the dichromated gelatine exposed alter in character in various ways, becoming less amenable to attack by various solvents and chemicals, for example.

The development referred to above involves treating the exposed plate to various chemicals in succession, for example as described above, although depending upon the desired effects, other chemicals such as hydrogen peroxide, alkali solutions, etc. may be used. One observable effect of such development process is to produce the variations in refractive index referred to above. As noted above, the exposed and developed material is characterised by minute voids, pores or pits, believed to be predominantly in the regions of the DCG which have not been exposed to light or have been only briefly exposed (see below).

Depending upon the specific process steps adopted and/or the treatment chemical employed, this"development"phase may effectively etch away entirely the regions between the fully exposed regions of DCG to leave, as illustrated in Figure 4, a series of separate columns 13 of hardened DCG, (where the mask used comprises an array of transparent holes or windows in an opaque background), or an array of wells or pits, each of a diameter corresponding with the diameter of a respective spot on a mask, (where the mask used comprises an array of opaque spots in a transparent background).

Alternatively, the configuration of the DCG layer may be, from a macroscopic viewpoint, unchanged, with the regions between the fully exposed portions of DCG being occupied by lower-density material comprising DCG with intervening voids and pores, e. g. of a spongy or honeycomb-like structure on a microscopic scale. In the latter case, the material may be subjected to a final chemical treatment the effect of which is to seal the upper surfaces of both the hardened and the spongy or porous DCG regions, so that the DCG layer has a smooth and impervious exposed surface such as indicated at 16 in Figure 3.

Alternatively, a coating of some other material may be applied which will dry or cure to form an impervious upper layer 16, or such coating may comprise a pre-formed film applied directly to the DCG.

Figures 5 and 6 each illustrate to a larger scale and somewhat schematically, a detail of the substrate/developed DCG layer of Figure 4 produced under respective slightly different conditions. In Figures 5 and 6, the shaded regions 13 above the substrate 10 represent the"hardened"portions of DCG which have been exposed to UV light. Depending upon the developing processes applied, the regions between these exposed regions may be entirely empty (as suggested by the"blank"representations of these spaces) or may be occupied by lower mean density, spongy, mat-like or honeycomb-like DCG material. Even in the latter case, it is believed that the developing (or etching) chemicals used are able to penetrate right down to the substrate 10 in the unexposed region and even to commence attacking, laterally, the flanks of the exposed regions, as indicated schematically in Figures 5 and 6 along the sides of these exposed regions. Because the DCG material itself to some extent acts as a filter to filter out ultraviolet, the intensity of the ultraviolet light, during exposure, is believed to be less in the regions closely adjacent the substrate 10 than adjacent the surface of the exposed DCG material and the consequent hardening effect to be similarly less, so that the lateral erosion of the pillars of exposed material, (or of the walls of the pits in the continuous matrix of exposed material, depending upon the nature of the mask used), is greater closer to the substrate 10 than further therefrom, as illustrated. Furthermore, as illustrated in Figures 5 and 6, the nature and depth of this lateral etching is believed to depend upon the intensity and length of the exposure of the DCG material so that more fully exposed and thus more completely hardened DCG material, as illustrated in Figure 6, is less susceptible to lateral attack by the etchant than less completely exposed material (illustrated in Figure 5). Figure 7 illustrates the possibility of producing DCG pillars or, alternatively, pits, which are inclined with respect to the major surface of the substrate, by exposing the DCG material by means of collimated ultraviolet light directed obliquely through the mask 14,15, (i. e. obliquely with respect to the plane of the substrate).

The region between broken lines indicated at 16 in Figures 5 and 6 represents the flowed gelatine surface referred to above, in the case where the blank regions shown on either side of each pillar of DCG 13 or within each"pit"in the DCG is a spongy or honeycomb-like gelatine structure. The layer 16 may, of course, be a separately formed sheet or foil.

In the case where the regions between the fully exposed regions 13 in Figures 5 and 6 are completely eliminated in the developing process, (i. e. to leave wholly empty spaces), the top layer indicated at 16 may comprise a separately formed continuous foil or film of gelatine or some other transparent material.

It will be understood that Figures 5,6 and 7, in particular, are to a very greatly enlarged scale. Thus, for example, the thickness of the DCG layer 12 and thus the height of the"pillars"13 illustrated in Figures 5,6 and 7, may be around 25p (micrometers) or less.

One novel and significant step in the process in accordance with the preferred embodiments of the invention is that the DCG layer is subjected to a brief "flash"blanket exposure to ultraviolet light prior to the selective exposure to ultraviolet through the optical mask. Some time is allowed to elapse between the flash exposure and the mask exposure. It is believed that the effect of the ultraviolet light upon the DCG material is to promote cross-linking in the gelatine. It is hypothesised that the effect of the initial"flash"exposure is to establish a network or mesh or mat of more strongly connected (by cross- linking) molecules with intervening microscopic regions being relatively unaffected. The mechanism is not well understood but some quantum effect may be involved. At any rate, it is hypothesised that the developing chemicals or etchants subsequently used, (after the full exposure to the mask) are able to penetrate, in the unexposed regions, between the fibres or chains of cross- linked molecules and effectively to remove the softer gelatine material whilst leaving, in place, a network or mat of pseudo fibres (linked molecules), which are densest adjacent the exposed surface of the DCG where the UV exposure has been strongest. During the final chemical treatment it is hypothesised that these fibres, adjacent the surface, are effectively fused together to form the continuous impervious outer layer 16.

As indicated above, it is contemplated that the porous or void-ridden exposed and developed DCG material might be used as a porous matrix to be impregnated with liquid crystal material and sandwiched between transparent electrode-coated plates of the kind usually used in liquid crystal displays. In such an arrangement, the fact that the liquid crystal material is finely broken up by the intervening structure of the porous DCG will significantly increase the response time of a display constructed in this fashion.

Thus, it is contemplated that an improved liquid crystal display might be made in the following fashion.

Referring to Figure 8, gelatine 20 is spread as a coating on top of the back plate of an LCD, which back plate may comprise, in manner known per. se, a glass plate having an electrode 23, for example of transparent electrically conductive mineral, on its upper surface. The gelatine layer 20 is dried and dichromated in the conventional way, the initial coating thickness being such as to achieve a desired coating thickness of around 5 microns. A layer of metal 24 is subsequently evaporated onto the surface of layer 20 and silica (SiO2) glass 26 is deposited in a layer over the metal. A photo-resist layer 28 is spun on top, exposed to light and etched to form a conventional mask for the Si02, (see Figure 9). The unwanted Si02 is then etched away, the surface cleaned up (to remove the photo-resist, etc.), and the SiO2 used as a mask during etching of the metal 24 away. The product illustrated at this stage in Figure 10 is then washed and cleaned and the DCG exposed to UV light through the metal mask. The result, after development of the material as described above, is a maze of "Chinese Walls"all over the surface of the LCD area, these"walls"comprising substantially non-porous hardened DCG with vacant"channels"or"channels filled with the porous or spongy gelatine, between such walls (such that the "channels"allow inflow of"liquid crystal"material and do not form closed cells). The remaining metal 24 and glass 26 thus provides a metal and glass cap to every such wall. The metal acts as the upper connection electrode for the respective LCD element. A glass top plate 30 is fitted on top of the assembly, as shown in Figure 11. This plate 30 can be a thin sheet of glass that will attach by"re-flowing"the deposited glass 26 on the top of the walls whilst in contact with the sheet 30. The open matrix of DCG (channels) between the back plate and the top plate may be filled with hot liquid crystal material in any convenient way.

In a variant embodiment, the DCG layer may be formed on, for example, a rotatable metal disk destined to form a platter of a computer hard disk drive. In this embodiment these minute cracks or pores in the DCG are filled with a so- called ferro-fluid, in any of several ways.

In the resulting structure, in which the DCG layer effectively incorporates a plurality of"cells"of fen-o-fluid, each such cell may in principle be utilise to record a binary'0'or a binary'1'by being magnetised in one direction or another. The individual cells may be written to by a conventional magnetic head such as used in disk drives or by laser beam or electron beam and may be read by a conventional magnetic head or by directing coherent light through the substrate and DCG coatings and detecting changes in the light transmitted, or by sensing changes in the pattern of light transmitted consequent upon polarisation changes dependent on the directions of magnetic polarisation of the individual cells of ferro-fluid.

A primary advantage of such a construction is that it allows a controlled reduction in the influence of each"magnetic cell", i. e. the ferro-fluid in each pore, upon its neighbours, thereby enabling faster response times and higher information-recording densities.

In variants, the DCG layer (s) may be replaced by a thin microporous layer of any other material, e. g. a polymer, which has adequate stability.

Figure 12 illustrates fragmentarily to a much enlarged scale a magnetic storage device of the kind referred to above, which may, inter alia, be used in a data store or memory in computer or microprocessor applications.

The product shown in Figure 12 preferably comprises a sheet or layer of a non- magnetic solid medium 42 having pores or cavities 44 therein occupied by a magnetic material 46 of which the state of magnetisation can be altered by an applied magnetic field, or by an applied laser light pulse or electron beam pulse, for example. The medium 42 may be supported on a rotatable disk 40, for example, to be incorporated in a magnetic disk drive.

In a preferred embodiment of the invention, the magnetic material 46 comprises a so-called ferro-fluid.

Preferably the solid medium 42 comprises a layer of dichromated gelatine, (or a series of superimposed layers of dichromated gelatine) on the supporting substrate 40, the DCG layer, in the manner explained above, having a plurality of minute pores or cracks extending from the surface of the DCG and constituting the pores or cavities 44. These cavities may be filled with the ferro-fluid by any convenient method.

According to another aspect of the invention there is provided a method of making a light-diffusing screen, e. g. for rear projection or for mounting in front of a pixelated LCD display to reduce or eliminate the perception of pixels, the method comprising providing a DCG layer on a transparent substrate, exposing the DCG layer to an image in, for example, ultraviolet light, comprising localised variations in light and shade and subsequently developing the DCG layer to establish corresponding refractive index variations in the DCG layer.

Preferably the exposure of the DCG layer to said image is effected by contact printing through a corresponding mask, as described above in relation to Figures 1 to 4 for example.

Preferably, the DCG is subjected to a blanket"flash"exposure, (i. e. an overall exposure of short duration) to ultraviolet light before being exposed to said image in ultraviolet light through the said mask, as also described above.

In rear projection applications, in particular, it is desirable to reduce as far as possible reflection of ambient light from the screen, in order to maximise contrast in the rear-projected image. It has been found that where a DCG layer is exposed and developed as discussed above, the regions which were previously shielded from exposure by the opaque regions of the mask take up dye preferentially, as compared with the exposed regions. Accordingly, in accordance with a preferred embodiment of the invention, the DCG layer, supported on its substrate, may be subjected to a dye, preferably a neutral- colour dye, during processing, so that the non-exposed areas are coloured darker by the dye, whereas the exposed areas are left substantially uncoloured.

By this means it is possible to make a rear projection screen with the capability of producing very high contrast in ambient light viewing conditions.

According to a still further aspect of the invention there is provided a reflector comprising a substrate carrying a thin layer of DCG, for example, a layer of 25 microns or less in thickness.

The applicants have observed that where, for example, a very thin layer of DCG is applied over, for example, a glass plate as a substrate, the combination behaves as an apparently perfect specular reflector for light striking the same, and being reflected therefrom, substantially normal to the plane of the reflector although for angles of incidence only a few degrees from the normal, and for greater angles of incidence, the reflective property disappears and the combination becomes substantially transparent to light. Highly efficient but inexpensive reflectors may be manufactured utilising this phenomenon, which may also, in accordance with the invention, be obtainable using certain photopolymers. It is hypothesised that the effect may be akin to that utilise in Lipmann photography. Preferably the thin DCG layer is exposed to coherent light directed in a parallel beam onto the DCG layer, preferably normal to the surface of the DCG layer, or at a small, preselected inclination to that normal.

According to a still further aspect of the invention, there is provided a diffuser comprising a layer of DCG on a supporting substrate, the DCG layer having been exposed to an optical image comprising localised variations in light and shade, such variations being preferably on a microscopic scale, the arrangement being such that the diffuser exercises a powerful diffusive effect for light striking the screen orthogonally and up to a predetermined angle of incidence, but is substantially transparent for greater angles of incidence, at least over a range of angles. The optical image may be produced by"contact printing" through a mask as described above, or by exposing the DCG to a projected or holographic image.

According to yet another aspect of the invention, there is provided a collimating diffuser, comprising a layer or sheet of dichromated gelatine or of a photopolymer exhibiting localised variations in refractive index such that, whilst light is diffused in passing through the screen, for each element or area of the screen, the polar distribution of light diffused by the element exhibits a mean deviation with respect to the direction of incidence of the incoming light reaching the screen such that the sheet or layer acts as a combined diffuser and lens.

Among the techniques which may be utilised to achieve this result are the exposure of the DCG 12 through the appropriate mask 14, 15 by light which, instead of being collimated parallel light, effectively radiates from a point source, (or an approximation thereto), indicated schematically at 60 in Figure 13, disposed at some distance from the mask and DCG layer and on the axis of the resulting diffusive"lens"so that in areas of the DCG layer or sheet adjacent the optical axis of the"lens", the light passes substantially normally through the layer whereas at greater radial distances from the axis, the light passes more obliquely through the mask and the DCG layer. Production of such a diffusive lens in this way is illustrated in Figure 13 where reference 12 again represents a DCG layer on transparent substrate 10, reference 60 represents the"point"light source and the arrows represent light emanating therefrom.

In accordance with a still further aspect of the invention, there is provided a depixelating screen, suitable for placing in front of a pixelated LCD display to remove or reduce the perception of individual pixels, the depixelating screen comprising a larger-scale two-dimensional or cross diffraction grating combined with a smaller-scale two dimensional or crossed diffraction grating or with a smaller-scale micro-feature, (e. g. microlens) array.

Figure 14 shows to a substantially enlarged scale, in plan, a portion of a diffusing or depixelating screen incorporating a larger-scale crossed grating, having a first set of crossed grating lines, indicated at 60, and comprising a group of equally spaced parallel lines extending in a first direction in the plane of the screen and a group of equally spaced parallel lines extending in the plane of the screen but perpendicular to the first mentioned direction, the screen additionally having a smaller-scale crossed grating, a microlens array, having a second set of lines, indicated at 62, comprising a group of lines extending in said first direction, but equally spaced much more closely, i. e. at a closer pitch, than the lines of said first set, and comprising a group of lines extending in said direction perpendicular to the first direction and likewise equally spaced much more closely than the lines of said first set. Alternatively, said second set of lines 62 may be replaced by rows and columns of microlenses, with the rows and columns again arranged at a much closer pitch than the set of lines 60. The grating lines 60,62, may be formed by grooves or ridges embossed in a transparent sheet, and in this case the grooves or ridges forming lines 60 may be deeper and/or wider than those forming lines 62 and are, at any rate arranged at a much coarser pitch. A corresponding grating may, of course, be formed by thicker and thinner crossed sets of lines, or in accordance with a preferred embodiment, the lines 60 and 62 may be formed as wide or narrower regions of refractive index different from the remainder of the material or, for example, the lines 60 might be formed as embossed grooves and the lines 62 might be defined between rows and columns of graded refractive index microlenses. In this case the diffuser may be formed using a photopolymer or DCG.